Visualization of Gas Transfer at Air-Sea Interface
by PIV-LIF Method
Nobuhito Mori
Department of Fluid Mechanics, Central Research Institute of
Electric Power Industry (CRIEPI),
1646 Abiko, Abiko, Chiba 270-1194, JAPAN.
Tel.:+81-471-821181, Fax.:+81-471-847142
Email: mori@criepi.denken.or.jp.
Keywords: Wind Wave, Air-sea interaction, Gas transfer, PIV, LIF
1
INTRODUCTION
An accurate estimation of gas transfer velocity at the air-sea interface is very important to understand environmental mechanisms of the earth. A wind dependent gas transfer model by Liss
and Merlivat model[1] has widely used the last decade. The gas transfer velocity of the Liss and
Merlivat model increases rapidly when the wind speed exceeds 13m/s. The rapid increase of gas
transfer velocity over 13m/s wind speed are explained by several reasons such as enhancements of
wind and breaking wave induced turbulence, breaking wave induced air bubbles, sea spray, and so
on. However, quantitative roles and mechanisms of the enhancement are not well known due to
the lack of detail measurements.
Particle image velocimetry (PIV) and laser-induced fluorescence (LIF) are powerful tools in
the study of turbulent flows. By coupling either PIV with LIF the turbulent transport processes
themselves can be directly measured through the calculation of the scalar concentration-velocity
correlations ui c . Several kinds of PIV and LIF coupling technique were demonstrated to measure
time-space variation of velocity and scalar fields[2][3]. LIF can measure pH through the calculation
of the differences of fluorescence between two different fluorescent material[3]. Using pH dependence
fluorescent dye, CO2 gas concentration can be measured because CO2 gas concentration has linear
dependence on pH.
The final goal of this study is to measure highly accurate gas transfer at the air-sea interface
under high speed wind conditions. In the study, the two-color LIF and PIV technique are employed
to visualize the water flows of the wind-wave fields.
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EXPERIMENTAL SETUP AND IMAGE PROCESSING
The experiments were conducted in a wind-wave flume that is 8.0 m long, 0.25 m wide, and 0.55 m
deep located in the Central Research Institute of Electric Power Industry (CRIEPI). The side walls
and bottom of the tank were constructed using acryl for optical access. The waves were generated
by a computer-controlled piston-type wavemaker with active absorber. An wave absorbing wall
was set up at the end of the tank. In the experiments the water depth, h, was kept at 25 cm.
The wave maker and fan were controlled by PC. The schematic diagram of experiment is shown
in Fig.1.
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Wave
generator
FAN
8.0m
Main Control PC
Figure 1: Experimental setup
CCD Cameras Frame Grabber
Air
Water
Cylindrical
Lens
Pulse Controller
YaG Laser
Main Control PC
Figure 2: Schematic view of measurement
The free surface elevation was measured using five capacitance-type wave gage and digital
images. Double pulse Nd:YaG laser with a wavelength of 532 nm was used as the light source. A
cylindrical lens mounted in front of the laser was used to create the light sheet. Three 10-bit CCD
cameras (RedLake ES1.0) with a resolution of 1K×1K pixels were used to capture the images. The
images were captured in a duration of 10s at a framing rate of 15 frames per second for LIF and 30
frames per second for PIV. Two CCD cameras were used for LIF and one CCD camera was used
for PIV. Each captured images was directly stored in individual PCs. The schematic illustration
of the cameras and control unit is shown in Fig.2.
Main control PC controlled the wind fan rpm and amplitude and frequency of the wave
generator. In order to understand the relationship between the flow pattern of the free surface
elevation, all the data was taken simultaneously and synchronized by main control computer. The
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base pulse was generated by pulse control PC at a rate of 15 per second. The emission of light of
YaG laser was triggered by base pulse. The duration of sequential pulses was 200µs. The schematic
diagram of pulses is shown in Fig.3.
The fluorescent sodium dye (FNa) and WT were used to visualize on the vertical plane of the
wind-wave flume. The fluorescence of fluorescein sodium is strongly pH sensitive at pH ¡ 7 and its
temperature dependence is weak. On the other hand, the fluorescence of WT is independent from
pH. The measured fluorescence of fluorescein sodium and WT in the experiments are shown Fig.4.
The WT is independent from pH, although fluorescein sodium is linearly dependent at pH¡7. For
PIV, 50µm polyethylene particles were mixed in the tank.
1.5
Base Trigger from Pulse Control PC
5V
1.125
CCD Exposure for LIF
I/I
pH=7
0V
FNa
WT
CCD Exposure for PIV
0.75
0.375
YaG Laser
0
2
4
T1
6
8
10
pH
T2
Exposure Time
Figure 4: Relationship between pH and fluorescein intensity of FNa and WT
∆t
Figure 3: Diagram of pulse, CCD exposure
and laser emission
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RESULTS AND DISCUSSIONS
LIF traditionally relies on a linear relationship between local fluorescence intensity F and local
dye concentration C. A simple model for this process is
F (x, y, t) = αA(x, y) I(x, y, t) C(x, y, t)
(1)
where α is fluorescent efficiency, A is the spatially dependent optical transfer function of the system,
I is the local light intensity. α and A(x, y) can be assumed as constant in the field. Thus, local
pH can be measured by
FF N a
[I(x, y, t) C(x, y, t)] F N a
=
(2)
FW T
[I(x, y, t) C(x, y, t)] W T
If FF N a /FW T is known as a function of pH or CO 2 concentration, simultaneous pH or CO2
concentration can be measured.
The uncertainty due to image distortion was also checked using fixed markers in the tank
and found that the maximum error was 1.5 mm (about 5 pixels). The free surface profile was
determined from the images by using the edge detection technique. The edge detection error is 2
pixels maximumly. Therefore, the error in measuring the free surface elevation is within 3 mm.
Fig.5 shows typical instantaneous velocity field superimposed on the simultaneously captured
normalized pH concentration for the case of wind speed u∗ =0.5m/s. The size of image is 200 mm
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Figure 5: Typical instantaneous velocity field superimposed over pH concentration (size of image
is 200X200mm).
× 200 mm. The top edge of figure indicates free surface, The downward bursting from the free
surface induce highly concentrated gas into deep water.
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CONCLUSIONS
A PIV-LIF experiments was conducted to measure the CO2 gas transfer at air-water interface
under in the wind-wave flume. The experimental result shows instantaneous gas concentration
and velocity field, quantitatively.
The detail of results of experiments will be discussed at the conference.
References
[1]P. Liss and L. Merlivat. Air-sea gas exchange: introduction and synthesis. he role of air-sea
exchange in geochemical cycling, pages 113–127, 1986.
[2]K. Hishida and J. Sakakibara. Combined plif-piv technique for velocity/scalar fields. Proceedings
of the third international workshop on PIV’99, 1:21–24, 1999.
[3]E.A. Cowen, K.-A. Chang, and Q. Liao. A single camera coupled ptv-lif technique. Experiments
in Fluids, 31:63–73, 2001.
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